The present invention relates to a suspension control apparatus for an automobile.
As a conventional suspension control apparatus, there can be mentioned an apparatus comprising a proportional solenoid valve having a movable member (spool) for controlling an amount of a hydraulic fluid passing therethrough according to a position of the movable member, the movable member being adapted to be displaced according to an intensity of a current applied to the solenoid; a variable damping force generating shock absorber provided between a vehicle body and an axle to generate a damping force which varies depending on the intensity of the current applied to the solenoid, that is, depending on the position of the movable member; and an acceleration sensor for detecting an acceleration of the vehicle body in a vertical direction. In this apparatus, the intensity of the current to be applied to the solenoid (output current) is determined, based on the detection value of acceleration of the acceleration sensor, to thereby generate a desired damping force (for an extension stroke and a compression stroke). Thus, damping of vibration of a sprung mass is effected, thereby improving the ride quality. As a shock absorber applied to the above-mentioned suspension control apparatus, there can be mentioned a shock absorber of a damping force reversing type in which when the damping force for the compression stroke is small, the damping force for the extension stroke changes from a large value to a small value and when the damping force for the extension stroke is small, the damping force for the compression stroke changes from a small value to a large value.
In the above-mentioned suspension control apparatus, the current to be applied to the solenoid is obtained in a form such that an oscillating current (dithering current, which is hereinafter frequently referred to simply as "dither") is superimposed on a command current having a desired intensity (a target current having an intensity equal to an average intensity of the current to be applied to the solenoid) which is determined from the detection value of acceleration of the acceleration sensor. The dithering current is obtained on the basis of a PWM (pulse width modulation) signal and has a frequency which is relatively high (but low as compared to the frequency of the PWM signal). Due to the dithering current, the movable member oscillates slightly (dithers) about a predetermined position corresponding to the intensity of the command current, so that displacement of the movable member can be easily performed, thereby improving responsivity in controlling a damping force.
A technique of superimposing a dithering current on a command current has been known and is disclosed in, for example, Unexamined Japanese Patent Application Public Disclosure No. 8-303628. From the viewpoint of achieving smooth displacement of the movable member, it is advantageous to slightly oscillate the movable member by utilizing a dithering current. However, a rate of change in damping force relative to slight changes in position of the movable member varies, depending on the position of the movable member. When the movable member is located at a position in a range such that the rate of change in damping force is high, a damping force changes by a large amount at a high frequency due to slight oscillation of the movable member (caused by the dithering current). This leads to a problem, such as generation of undesirable noise and vibration.